by 
The Curious Bag
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<h4 class="dechead">On One Hand: Minimal Wasted Power
Leaving your cellphone plugged in is convenient and wastes only a tiny amount of power. Cellphone chargers suck in much less power when they are not plugged into a phone than when they are. According to a 2007 study in the Universities Power Engineering Conference publication, while chargers use between 1 and 2 watts of power while connected to a phone, they use less than 200 mW when completely idle.
On the Other: Larger Effects
However, these small power amounts add up dramatically when used over a long period of time and over a large scale. A task force led by Nokia estimated that if 10 percent of the world's cellphone users unplugged their chargers when not in use, it would save enough energy to power 60,000 homes per year in Europe.
Bottom Line
Although your individual power usage may not change drastically when you unplug your charger, a collective effort by enough cellphone users would have a significant effect on energy consumption.
Source:
Unplug Your Cell Phone Charger
Calorimetry for Power Conversion in Mobile Telephone Chargers
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